1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer, and a digital copying machine, and an optical beam scanning device to be used for an image forming apparatus.
2. Description of the Related Art
Conventionally, the optical beam scanning device has been applied widely as for example an optical system for supplying a laser beam to an image forming section for forming a latent image on a photosensitive member based on image data in an image forming apparatus such as a laser printer and a digital copying machine.
Such an optical beam scanning device comprises a semiconductor laser element (laser diode) as a light source, a (light sources side) pre-deflection optical system for narrowing the cross-sectional shape of a laser beam output from the laser diode or the like to a predetermined size, a scanning optical system for deflecting the narrowed laser beam by a rotatable polygon mirror in the photosensitive member (photosensitive drum) rotation axis direction, that is, in the main scanning direction, an (image plane side) post-deflection optical system for focusing the deflected laser beam substantially equally at any position along the main scanning direction of the image plane of the photosensitive member, or the like. The direction orthogonal to the main scanning direction of the image plane is defined to be the sub scanning direction.
According to the optical beam scanning device, it is desirable to restrain irregularity of the light amount reaching onto the image plane of the photosensitive member to a small degree. Conventionally, various methods for restraining the irregularity of the light amount to a small degree have been proposed.
As a kind of the method for restraining the irregularity of the light amount to a small degree, there is a method of utilizing the laser beam polarization direction. As a specific method thereof, there is a method of providing the polarization direction between a P polarized light beam and an S polarized light beam, or as a circularly polarized light beam.
For example, the method disclosed in the official gazette of the Japanese Patent Application Laid-Open (JP-A) No. 2001-91873 is as follows. That is, the angle θ formed by the polarization direction of the diverging light from the laser diode and the direction perpendicular to the reflection surface of the rotatable polygon mirror is provided as 10° ≦θ≦80°.
However, according to the method of providing the polarization direction between a P polarized light beam and an S polarized light beam, or as a circularly polarized light beam, since the phase difference is generated by the P wave component and the S wave component, depending on the incident angle in the case the laser beam polarization direction is incident on the metal rotatable polygon mirror surface, an elliptical polarization with the major axis direction (angle) changed according to the angle change of the rotatable polygon mirror is provided. Therefore, it is difficult to constantly maintain the light amount. Moreover, according to an ordinary lens, the birefringence with the refractive index provided differently depending on the polarization direction is generated due to the influence such as an internal distortion at the time of shaping, processing, or the like. In the case the polarization direction is changed, the light focusing position, or the like is changed as well so that the image forming characteristic is deteriorated as a result, and thus a problem is involved in that the beam cannot be focused, flare is increased, or the like.
One with an adjustable polarization direction has been proposed as well. However, an adjusting mechanism is needed so as to give rise to cost increase, such as the need of the steps for adjustment. Moreover, in general, the beam divergence angle of the laser diode differs depending on the direction, and thus an elliptical beam is provided. Therefore, in the case the laser diode angle itself is rotated around the optical axis, the beam size on the surface to be scanned itself is influenced.
Furthermore, in the case light beams from a plurality of light sources are utilized like in a color image forming apparatus, not only the light amount irregularity of the scanning lines should be restrained but also the light amounts in the scanning lines should be equal. However, as a conventional method, a method of utilizing semiconductor lasers of the same characteristic, or the like has been provided, and the apparatus range with the method usable has been limited.